1. Field of the Invention
The present invention relates generally to an ohms converter circuit for measuring the resistance of an unknown resistor, and more particularly pertains to an ohms converter circuit employing a highly stable current source which is protected against the accidental application thereto of high voltages.
2. Discussion of the Prior Art
In a digital multimeter, an ohms measurement is commonly performed by measuring the voltage drop produced across an unknown resistor with the application thereto of a known current flow. In order to get an accurate reading, two measurements must be performed simultaneously, namely the voltage drop across and the current flow through the unknown resistor. This measurement technique generally utilizes either a sample and hold circuit or two A/D converters. However, both approaches are generally unacceptable because mismatches and drift create errors in the measurements.
In lieu of performing simultaneous measurements, the approach of an ohms converter circuit can be employed, which utilizes a highly stable current source. Most ohms converter circuits regulate the voltage drop across the emitter resistor of a transistor, the collector of which is the output. With this approach, the beta of the output transistor changes with temperature and with changes of V.sub.CE, which causes I.sub.C to change, thereby creating an inherent error. The ohms converter of the present invention eliminates this basic design flaw by monitoring the actual output current of the current source.